1. Field of the Invention
This invention relates in general to an improvement in a steering system of a vehicle and more specifically to an improvement in a steering system comprising a power steering system for assisting in turning front wheels under the action of hydraulic fluid and a rear wheel control system for controllably turning rear wheels.
2. Description of Prior Art
In order to improve cornering performance of modern automotive vehicles, a steering system by which the controllable turning of both front and rear wheels are possible has been proposed, which system is disclosed as a compliance steer control system, for example, in U.S. Pat. No. 4,440,254 issued on Apr. 3, 1984 to Shibahata et al.
This compliance steer control system is equipped with hydraulic actuators associated with elastomeric insulators interposed between a rear suspension arrangement and a vehicle body. The hydraulic actuators are supplied with hydraulic pressure to be fed to a power cylinder of a power steering system, thus achieving a control to increase or decrease so-called compliance steering of the rear wheels during cornering of the vehicle. This control system arrangement is intended mainly to improve vehicle cornering performance at high vehicle speeds by turning the rear wheels in the same direction as the front wheels upon turning a steering wheel to set vehicle cornering characteristics on the understeer side.
Now, as is well known, in order to provide an appropriate steering feeling to a driver, the conventional power steering system is arranged such that steering assist power developed by the power cylinder is minimized at high vehicle speeds and no steering assist power is developed when the driver's steering effort applied to the steering wheel is very slight. This arrangement provides a so-called non-sensitive range in which no steering assist power is developed. Accordingly, the power cylinder of the power steering system is supplied with low hydraulic pressure during high vehicle speeds and is supplied with no hydraulic pressure when a very slight turning effort is applied to the steering wheel.
However, in the compliance steering control of rear wheels, it is desirable to cause a greater compliance steering to turn the rear wheels large amounts during high speed cornering for the purpose of attaining more stable cornering. Additionally, it is also desirable to turn the rear wheels even upon a slight turning angle of the front wheels or a slight steering effort applied to the steering wheel being slight when the steering wheel is rotated. In this regard, it has been extremely difficult to operate the conventional compliance steer control system so as to obtain optimum characteristics for cornering performance wherein the hydraulic actuators thereof are supplied with the hydraulic pressure fed to the power cylinder of the power steering system. As a result, it becomes necessary to control the hydraulic pressure supplied to the compliance steer control system, independently from the hydraulic pressure supplied to the power cylinder of the power steering system.
In this regard, even if individual control valves are respectively provided for the power steering system and for the compliance steer control system which are different from each other in operational characteristics, their desired function will not be attained, particularly because these valves are of the type wherein pressurized hydraulic fluid discharged from an oil pump is fed back to a reservoir when the steering wheel is in its neutral position. More specifically, in a power steering system with a non-sensitive range as stated above, when hydraulic fluid is fed to the hydraulic actuators of the compliance steer control system upon a slight steering effort being applied to the steering wheel, hydraulic circuit flow resistance on the compliance steer control system side increases and therefore hydraulic fluid discharged from the oil pump is unavoidably fed back to the reservoir through the control valve for the power steering system. As a result, an insufficient amount of hydraulic fluid will be supplied to the hydraulic actuators of the compliance steer control system.